Cassini UVIS observatory
phase spectral imaging of the
Saturn system
D. Shemansky(1)
Cassini UVIS team(2 – 6)
PI: L. Esposito
(1)University of Southern California, (2) University of Colorado,
Boulder, (3)Max Planck Inst (MPIS), (4)JPL, (5) C. Az. Coll.,
(6)Calif. Inst. Technology
History of neutral gas
measurements and analysis
• Atomic hydrogen predicted by McDonough
and Brice (1973) as a torus at Titan orbit
• Detected by Weiser et al. (1975) with JHU
rocket. Undetermined location, but assumed
to be at Titan orbit.
• Voyager UVS: Broadfoot et al. (1977).
Interpreted as a torus.
•Sittler et al. (1981): Report energetic electron depletion in 2.8 RS to
3.7 RS region and infer possible neutral gas as absorber. Identify
separate absorption effect of a larger E-ring presence.
•Hood (1989): Found large losses of energetic protons in the 5 to 12
RS region inadequately explained by then known gas abundances or
ring particles.
•Shemansky and Hall (1992): Analyzed Voyager 1 data, concluding
that the entire magnetosphere is filled with atomic hydrogen
merging into the top of the Saturn atmosphere. Energy sink analysis
of the very low electron temperature in the plasma sheet reported by
the PLS experiment is used to predict large abundance of the neutral
oxygen and hydroxyl radical water dissociation products.
S&H(1992) Voyager 1 Saturn atomic hydrogen
Cassini UVIS image of atomic
hydrogen in the Saturn system
Cassini UVIS image of atomic hydrogen in the
Saturn system
History continued
• OH discovered in the 3 – 6 RS region of the
Saturn magnetosphere using HST
[Shemansky et al., 1993]
• OI discovered using Cassini UVIS imaging,
Dec 25, 2003 – May 13, 2004.
Simulation of core neutral gas distribution
OH (from HST) vs OI torus
OH
OI
Total neutral gas abundance
• atomic hydrogen: 2 X 1035 atoms
Loss rate: > 1.3 X 1027 atoms s-1
• atomic oxygen: 3 – 4 X 1034 atoms
Loss rate: ~ 1028 atoms s-1
Characteristics of UVIS
observations
• The abundance of OI increased from
Dec/Jan to a peak in Feb/March and
declined to slightly less than the original
value by mid May.
• The OI distribution in the orbital plane
expanded outward in Feb/March and
contracted inward in following months.
• The peak emission shows a distinct
asymmetry in the early months, peaking on
the planet darkside.
Implications of UVIS
observations
• The rise and fall in abundance of OI
between Dec 25, 2003 and May 13, 2004
amounts to at total inferred ~500 Mkg of
mass apparently lost from the system in
this period. Mean loss rate of bulge in mass
during 2 months is ~4 X 1027 atoms s-1.
• Total mass of OI + OH in system is ~2200.
Mkg.
• The estimated mass of micron sized
particles in the E-ring involved in the Mimas
– Tethys region is 600 Mkg.
Implications continued
• Using the plasma sheet model
(Richardson, 1997) based on the Voyager
PLS measurements, the charge capture
characteristic time for loss of OI is 40
days. This independent rate process loss
rate also indicates that mass equal to the
micron sized E-ring particles is lost from
Saturn in 1 to 2 months.
• Including the projected unobserved ~5 km
E-ring objects, the lifetime of the entire Ering against this process is ~100 M years.
Proposed primary water product
plasma reactions:
Neutral gas loss processes:
O + O+  O+ + O
OH + O+  OH+ + O
H+ + O  O+ + H
O+ + H  H+ + O
Reactions continued
Neutral Gas production:
Plasma loss: Cluster-ion formation
O2H2+.(H2O)n.G + e  O2 + H2 + G
O2H2+.(H2O)n.G + e  O + H + OH + G
O2H2+.(H2O)n.G + e  O2 + H + H + G
H3O+. (H2O)n.G + e  OH + H2 + G
H3O+.(H2O)n.G + e  O + 3H + G
Neutral and plasma loss
OH+ + e O + H
Conclusions
• The UVIS imaging of the OI population in the
magnetosphere has revealed a very complex
system that is eroding icy solids at a high rate. The
required large exposed surface area implies micron
and smaller particles are the primary reaction
targets.
• We propose that low energy plasma ions forming
solid state cluster ions may be the primary neutral
gas source process and plasma quenching
reaction.
• A single stochastic event has been observed, that
may represent one erosion mechanism.
Conclusions continued
• The required energy deposition for the
erosion process (~14gW) is plausibly
moderately energetic magnetospheric
electrons, that derive their energy from the
rotating magnetic field.
• The Saturn magnetosphere is completely
different from that of Jupiter.
Jupiter neutral/ion = ~0.003
Saturn neutral/ion = ~100.